• A A Bidokhti

      Articles written in Journal of Earth System Science

    • Estimation of urban mixed layer height in Zanjan using LIDAR observations and numerical modeling

      A A Bidokhti M Khoshsima S Sabetghadam H M Khalesifard

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      Air pollution predictions often require the height of atmospheric mixed layer in time especially in big cities. Here, the variation of the height of this layer is estimated from direct measurements and also from a numerical forecast model with a high resolution boundary layer scheme. The height of the daytime mixed layer for the city of Zanjan (48.5°N, 36.7°E, 1700 m above sea level)is measured using a LIDAR (532 nm)system, which works based on aerosols scattering of laser light. The mixed layer height ($z_i$) for Zanjan city, well above mean sea level compared to other major cities in the world,is found to be between 1.4 km typically in spring and 2.2 km in summer, for synoptic calm conditions. Also, the MM5 forecast model with a proper boundary layer scheme (MRF)is used to estimate $z_i$ which shows rather good agreement with direct observations using the LIDAR system. The entrainment zone of the mixed layer was also found to undergo some occasional temporal growth that may be attributed to shear instability that led to more mixed layer growth.

    • Study of vertical wind profiles in an urban area with complex terrain (Tehran)

      N Pegahfar A A Bidokhti P Zawar-reza M M Farahani

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      In this paper monthly trends of vertical wind profiles within and above an urban area with complex topography (Tehran) have been investigated using data from a Sodar, a 100-m and two 28-m towers and some surface stations. It includes the occurrence, evolution, dissipation time, peak time and maximum height for katabatic–anabatic winds due to topography, return flow and urban circulation. Vertical distributions of wind and the heat and momentum fluxes up to 600 m were also considered.

      The vertical wind profiles which have diurnal and seasonal variations show southwesterly daily anabatic wind and northeasterly nocturnal katabatic wind. Daily vertical wind profile structure which has two layers and two jets contains a decrease approximately at 300–400 m which may be due to the return flow and a daily thermal convective flow in the urban convergent terrain. At night the nocturnal wind profile structure, in the majority of months, has two layers and sometimes three layers containing northeasterly flows. In two layers structure, a decrease can be seen between two layers. In three layers structure, the middle layer has a different direction that indicates the return flow and urban circulation more clearly. Katabatic flows also develop in successive phases varying from 1 to 3 phases in different months. Investigation on surface wind demonstrates that in cold months a drainage flow from a valley located in the west of the station can affect wind speed and direction especially at evening transition time. These flow patterns can influence the way air pollutants disperse in this area.

    • Variations of aerosol optical depth and Angstrom parameters at a suburban location in Iran During 2009–2010

      M Khoshsima A A Bidokhti F Ahmadi-Giv

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      Solar irradiance is attenuated spectrally when passing through the earth’s atmosphere and it is strongly dependent on sky conditions, cleanliness of the atmosphere, composition of aerosols and gaseous constituents. In this paper, aerosol optical properties including aerosol optical depth (AOD), Angstrom exponent (𝛼) and Angstrom turbidity coefficient (𝛽) have been investigated during December 2009 to October 2010, in a suburban area of Zanjan (36°N, 43°E, 1700 m), in the north–west of Iran, using meteorological and sun photometric data. Results show that turbidity varies on all time scales, from the seasonal to hourly, because of changes in the atmospheric meteorological parameters. The values of 𝛼 range from near zero to 1.67. The diurnal variation of AOD in Zanjan is about 15%. The diurnal variability of AOD, showed a similar variation pattern in spring (including March, April, May) and winter (December, January, February) and had a different variation pattern in summer (June, July, August) and autumn (September and October). During February, spring and early summer winds transport continental aerosols mostly from the Iraq (dust events) and cause the increase of beta and turbidity of atmosphere of Zanjan.

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